High gas velocity start-up of an ethylene cracking furnace

ABSTRACT

In chemical processes for cracking hydrocarbons, reactors are subject to coking During the decoke process carburization of the metal substrate can occur, negatively impacting reactor life. Decokes are also costly due to down-time where costs are incurred without production of commercial products. Reducing the frequency of decokes provides an opportunity to reduce the financial impacts of downtimes. A start-up procedure is described herein that limits initial coke deposition, leading to a reduced tendency for carburization of the metal substrate, improving reactor life, and more importantly, extending reactor run length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/964,243, filed Jan. 22, 2020, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the field of hydrocarbon cracking furnaces. In particular, the present disclosure relates to start-up procedures for hydrocarbon cracking furnaces.

BACKGROUND ART

In an industrial steam cracker, a feed is passed through several reactors or “furnaces”, each furnace including a radiant section with tubular metal coils, before exiting the furnace at an elevated temperature, typically above 750° C. At these temperatures the steam and feed, typically an alkane, usually a lower molecular weight alkane such as ethane, propane, butane and mixtures thereof, or heavier feed stock including naphtha, heavy aromatic concentrate (HAC) and heavy aromatic gas oil (HAGO) or any of the vacuum gas oils, undergoes a rearrangement yielding alkenes, including but not limited to ethylene, propylene, and butene, as well as hydrogen and other co-products. Over time, heavy hydrocarbons and coke build up on the internal surface of the radiant tubes increasing the pressure drop across the tube and reducing the thermal and cracking efficiency process in that tube or coil. The furnace is taken off-line (hydrocarbon feed is no longer passing through the coil) and the coil(s) is decoked after which the furnace is returned to operation.

Decokes can be costly as they represent time periods where costs are incurred without production of commercially valuable products. Reducing the time to perform a decoke can save money, but options for doing so are limited. Increasing the time between decokes by increasing the run length is another option for reducing the financial effect of the costly downtime. Provided herein is a start-up method for a hydrocarbon cracking furnace that can extend run length.

SUMMARY OF INVENTION

The present disclosure seeks to provide a start-up procedure to extend a run length of a hydrocarbon cracking furnace, the hydrocarbon cracking furnace including at least one furnace tube, wherein the at least one furnace tube includes at least one inlet, at least one outlet, and a point of incipient cracking in between the inlet and outlet, the start-up procedure including: introducing a fluid including hydrocarbon and dilution steam to the at least one inlet; establishing and maintaining the fluid at the inlet to a temperature of between 25° C. and 225° C.; determining a number average molecular weight of the fluid proximate the inlet; determining a number average molecular weight of the fluid proximate the outlet; calculating an average of the number average molecular weights of the fluids at the inlet and the outlet; measuring a pressure drop of the fluid from the inlet to the outlet; calculating a fluid velocity at the point of incipient cracking; controlling the fluid velocity at the point of incipient cracking to a range between at least 90 to at most 115 m/s for at least 5 days by varying the flow rate and/or one or more of the fluid properties of the fluid at the inlet; wherein the start-up procedure extends the run length by at least 15% relative to a start-up procedure conducted in the same manner but wherein the fluid velocity at the point of incipient cracking is not monitored and/or controlled.

The present disclosure also seeks to provide a start-up procedure to extend a run length of a hydrocarbon cracking furnace, the hydrocarbon cracking furnace including at least one furnace tube, wherein the at least one furnace tube includes at least one inlet, at least one outlet, and a point of incipient cracking in between the inlet and outlet, the start-up procedure including: introducing a fluid including hydrocarbon dilution steam to the at least one inlet; establishing and maintaining the fluid at the inlet to a temperature of between 25° C. and 225° C.; determining a number average molecular weight of the fluid proximate the inlet; determining a number average molecular weight of the fluid proximate the outlet; calculating an average of the number average molecular weights of the fluids at the inlet and the outlet; measuring a pressure drop of the fluid from the inlet to the outlet; calculating a fluid velocity at the point of incipient cracking; controlling the fluid velocity at the point of incipient cracking to a range between at least 90 to at most 115 m/s for at least 5 days by varying the flow rate and/or one or more of the fluid properties of the fluid at the inlet; wherein the start-up procedure extends the run length by at least 8 days relative to a start-up procedure conducted in the same manner but wherein the fluid velocity at the point of incipient cracking is not monitored and/or controlled.

BRIEF DESCRIPTION OF DRAWINGS

To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced.

FIG. 1 illustrates a steam hydrocarbon cracking furnace 100 layout in a schematic process flow diagram in accordance with one embodiment.

FIG. 2 illustrates a steam hydrocarbon cracking furnace 200 layout in a schematic process flow diagram in accordance with one embodiment.

FIG. 3 illustrates a steam hydrocarbon cracking furnace 300 layout in a schematic process flow diagram in accordance with one embodiment.

FIG. 4 illustrates a steam hydrocarbon cracking furnace 400 layout in a schematic process flow diagram in accordance with one embodiment.

FIG. 5 illustrates a graph of the gas velocity in an industrial steam hydrocarbon cracking furnace over a 500 day period which includes 3 separate runs.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to certain embodiments of the disclosed subject matter. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter.

Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the properties that the present disclosure desires to obtain. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Definitions

“Aromatic hydrocarbon” refers to a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle.

“C1-C4 alkane” refers to one or more of methane, ethane, propane, and butane.

“Dilution steam” refers to steam added to the hydrocarbon to be cracked in a hydrocarbon cracking furnace.

“Fluid properties” refers to properties of the fluid, including, but not limited to, density, viscosity, temperature, pressure, specific volume, specific weight, specific gravity, and number average molecular weight.

“Furnace tube” refers to a conduit, often described as a pass and where multiple passes are linked together in what is typically referred to as a coil, that can be used in a furnace, through which the fluid to be heated flows.

“Heavy aromatic distillate” refers to a combination of hydrocarbons obtained from distillation of aromatic streams. It consists predominantly of aromatic hydrocarbons having carbon numbers predominantly in the range of C9 through C16 and boiling in the range of approximately 165° C. to 290° C. It can be a co-product from ethylene production. (also known as HAD)

“Hydrocarbon” refers to an organic compound consisting entirely of hydrogen and carbon.

“Hydrocarbon cracking furnace” refers to a furnace designed to break down or crack hydrocarbons, typically alkanes into alkenes.

“Ideal Gas Law” refers to the equation of state of a hypothetical ideal gas; it is a good approximation of the behavior of many gases under many conditions and may be used in place of measuring or calculating the actual properties of a gas. The Ideal Gas Law is often written as: PV=nRT, where P, V and T are the pressure, volume and temperature, n is the number of moles of gas, and R is the ideal gas constant.

“Naphtha” refers to a flammable liquid hydrocarbon mixture. Mixtures labelled naphtha have been produced from natural gas condensates, petroleum distillates, and the distillation of coal tar and peat. In different industries and regions naphtha may also be crude oil or refined products such as kerosene.

“Number average molecular weight” refers to the total weight of the sample divided by the number of molecules in the sample.

“Point of incipient cracking” refers to the location in the hydrocarbon cracking furnace where the hydrocarbon starts to crack, forming radicals. This location is often approximated, and is based on many factors, including the pressure in the hydrocarbon cracking furnace, feed composition, coke formation, desired products, etc. The temperature at the point of incipient cracking can range as low as 370° C. to as high as 850° C. in industrial furnaces, typically 500-550° C. For this disclosure, the location where the temperature first reaches 525° C. was chosen as it is typical temperature for a furnace cracking a predominately ethane feed. The pressure at the point of incipient cracking was determined using SPYRO® Suite 7 software from the Pyrotec Division of Technip Benelux B.V.

“Pressure drop” refers to the difference in total pressure between two points of a fluid carrying network.

“Run length” refers to the length of time that a furnace tube is in hydrocarbon cracking operation.

“Start-up procedure” refers to the steps followed to start a hydrocarbon cracking furnace until it is producing the desired alkenes.

One method of producing alkenes such as ethylene is by steam cracking, in which hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons, by thermally cracking with the use of dilution steam in a bank of pyrolysis furnaces. The starting hydrocarbons can be C1-C4 alkanes, naphtha, or aromatic hydrocarbons such as heavy aromatic distillate (HAD).

In an industrial hydrocarbon cracking furnace, there are typically several reactors or coils. Within the radiant section of each furnace are tubular metal coils or furnace tubes with one or more zones with one or more passes per furnace tube which proceed through a furnace where the fluid within exits the furnace tube at an elevated temperature typically above 750° C., usually in the range of 800° C. to 900° C., the feed passing through the furnace tube in the radiant section of a cracker for a period of time from milliseconds to several seconds. The one or more zones can be included of a convection section or pre-heat section and a radiant cracking section. After the fluid exits the radiant cracking section, it can be sent to a quench section to lower the fluid temperature. The temperature of the fluid entering the pre-heat section is typically between 25° C. and 225° C., whereas the temperature of the fluid entering the radiant section is typically between 600° C. and 750° C. The temperature at the point of incipient cracking can range as low as 370° C. to as high as 850° C. in industrial furnaces, typically 500-550° C. at typical pressures and compositions, with 525° C. being used in this disclosure in all calculations. At these temperatures the dilution steam and hydrocarbon feed, typically an alkane, typically a lower molecular weight alkane such as ethane, propane, butane and mixtures thereof, or heavier feed stock including naphtha, heavy aromatic distillate (HAD) and heavy aromatic gas oil (HAGO) or any of the vacuum gas oils, undergoes a rearrangement yielding alkenes, including but not limited to ethylene, propylene and butene as well as hydrogen and other coproducts.

Over time, heavy hydrocarbons and a carbonaceous deposit, or coke-like material, build up on the internal surface of the furnace tube increasing the pressure drop across the furnace tube and reducing the thermal and cracking efficiency process in that furnace tube. Coking is an unwanted side reaction from hydrocarbon cracking. It is a major operational problem in the radiant section of hydrocarbon cracking furnaces and transfer line exchangers. Dilution steam lowers the hydrocarbon partial pressure of the cracked compounds and reduces the tendency of coke deposition on the furnace tubes. Dilution steam increases the distance between fouling components and can be used to increase the velocity of the gas by adding its volume to the flow.

Coke is an undesired but inevitable side product of the pyrolysis. Surface catalyzed reactions lead to the formation of filamentous coke. In many cases, the coke formation is caused by nickel and iron on the alloy surface. Amorphous coke is formed in the gas phase. Increased pressure drop, impaired heat transfer and higher fuel consumption due to the coke cause high production losses. The external furnace tube skin temperature can also rise, which influences the process selectivity and leads to even more rapid coke formation. The formed coke must be removed by controlled reaction with steam and air. It is a non-productive downtime of the steam cracker furnace. Decoking cycles lead to shorter coil life of the steam cracker furnaces. During a decoking cycle, the furnace is taken off-line (i.e., hydrocarbon feed is no longer passing through the furnace tube) and the furnace tube is decoked, after which the furnace is returned to operation.

The present disclosure relates to field of hydrocarbon cracking furnaces. In particular, the present disclosure relates to start-up procedures for hydrocarbon cracking furnaces. The start-up procedures can be implemented for the first start-up of the hydrocarbon cracking furnace, after a decoking procedure, or any other time the hydrocarbon cracking furnace is starting up. The hydrocarbon cracking furnaces are typically taken off-line or may require being shut down on a periodic basis to remove coke accumulated on the internal surfaces of the furnace tubes. The present disclosure is suitable for any cracking process where dilution steam with hydrocarbon molecules are converted to other hydrocarbon molecules at elevated temperatures where coke is a byproduct on the furnace tubes or reactors, such as a fluid catalyst cracker or a steam cracker, used to produce alkenes from corresponding alkanes at elevated temperatures.

The present disclosure seeks to provide a method for managing initial coke deposition on a hydrocarbon cracking furnace's tube wall inner surface by maximizing boundary layer turbulence of the hydrocarbon and any diluent present. The greater the velocity at the temperature in the furnace tube where cracking begins, the greater the scrubbing action of the turbulent flow, which minimizes the amount of coke laydown onto the furnace tube of a newly built, rebuilt, or de-coked hydrocarbon cracking furnace. The high gas velocity minimizes the thickness of the laminar flow of the boundary layer at the furnace tube wall providing a scouring effect upon the newly formed and typically deposited coke.

FIG. 1 shows a schematic drawing of a hydrocarbon cracking furnace 100. In this steam cracking furnace version, a dilution steam 102 is combined with a hydrocarbon feed 104 to be cracked. The combined fluid enters a pre-heat section 106. The fluid continues through the apparatus into a radiant section 108. The fluid now contains dilution steam, some of the initial hydrocarbon feed and newly made cracked gas. Then the fluid enters a quench section 110.

FIG. 2 shows a schematic drawing of a hydrocarbon cracking furnace 200. In this steam cracking furnace version, a dilution steam 202 is combined with a hydrocarbon feed 204 to be cracked. The dilution steam 202 is preheated in a dilution steam pre-heat section 212. The combined fluid enters a pre-heat section combined stream pre-heat section 214. The dilution steam pre-heat section 212 and the combined stream pre-heat section 214 make up a pre-heat section 206. The fluid continues through the apparatus into a radiant section 208. The fluid now contains dilution steam, some of the initial hydrocarbon feed and newly made cracked gas. Then the fluid enters a quench section 210.

FIG. 3 shows a schematic drawing of a hydrocarbon cracking furnace 300. In this steam cracking furnace version, dilution steam 302 is combined with hydrocarbon feed 304 to be cracked. The dilution steam 302 is preheated in a dilution steam pre-heat section 312 and hydrocarbon feed 304 is preheated in a hydrocarbon feed pre-heat section 314. The combined fluid enters a combined stream pre-heat section 316. The dilution steam pre-heat section 312, the hydrocarbon feed pre-heat section 314 and the combined stream pre-heat section 316 make up a pre-heat section 306. The fluid continues through the apparatus into a radiant section 308. The fluid now contains dilution steam, some of the initial hydrocarbon feed and newly made cracked gas. The fluid enters a quench section 310.

FIG. 4 shows a schematic drawing of a hydrocarbon cracking furnace 400. In this steam cracking furnace version, dilution steam 402 is combined with hydrocarbon feed 404 to be cracked. The dilution steam 402 is preheated in a dilution steam pre-heat section 412. The hydrocarbon feed 404 is preheated in a hydrocarbon feed pre-heat section 414. The hydrocarbon feed 404 to be cracked joins the dilution steam 402 on the outside of hydrocarbon cracking furnace 400. The combined fluid re-enters the combined stream pre-heat section 416. The dilution steam pre-heat section 412, the hydrocarbon feed pre-heat section 414 and the combined stream pre-heat section 416 make up a pre-heat section 406. The fluid continues through the apparatus into a radiant section 408. The fluid now contains dilution steam, some of the initial hydrocarbon feed and newly made cracked gas. The fluid enters a quench section 410.

Decoking

In decoking a steam cracker furnace several different methods are available. In one method, the coke is physically scoured from the internal furnace tube walls. Typically, a relatively high velocity stream of air, steam or a mixture thereof passes through the furnace tube resulting in small particulate materials being included in the effluent stream. As the particulates pass through the furnace tube, the coke on the internal wall is scoured or eroded from the wall. One issue with this type of treatment is the erosion of the internal surface of the furnace tube, fittings and downstream equipment. An additional concern with this type of treatment is downstream plugging with coke particulates scoured from the furnace tube walls.

An alternate treatment to decoke the furnace tube is to react or “burn” the carbon accumulation from the furnace tube wall. When the furnace is taken off-line, air and steam are passed through the furnace tube at an elevated temperature to cause the coke to react or burn. The progress of the decoking process may be measured in several different ways including measuring the carbon dioxide and carbon monoxide content in the effluent gasses leaving the furnace, measuring the furnace tube metal temperature, measuring changes in the furnace tube outlet pressure or changes in the furnace tube fouling factors. A mixture of steam and air is passed through the coil while it is maintained at an elevated temperature from about 750° C. to about 900° C., in some embodiments from 780° C. to 850° C. in some embodiments from 800° C. to 830° C. The amount of air fed to the tube or coil depends on the furnace and the coil design. In some instances, the air may be fed to the coil at a rate from about 10 kg/hr to about 1000 kg/hour. Steam is fed to the reactor to provide an initial weight ratio of steam to air from about 200:1 to about 170:3. The decoke is completed when the amount of gasified carbon (CO₂ and CO) in the effluent stream from the tube or coil is below about 2,000 ppm of CO₂. In some embodiments of the procedure, the rate of air feed to the coil may be increased up to about 1000 kg/hr/coil as a post burn and/or as a surface polishing step.

During the decoke procedure the temperature in the combustion side of the cracker (sometimes called the radiant box or zone) may range from about 760° C. to about 1100° C.

The rate of decoking needs to be controlled to minimize or limit spalling of coke from the inner surface of the coil as this may interfere with downstream operation. Also during decoking, the external temperature of the tube should be maintained as uniform as possible to prevent damage to the tube.

The decoking may be finished with a steam scour at a steam feed rate of not less than 3500 kg/hr/reactor for a time from 0.5 to 10 hours, in some embodiments from about 6 to 9 hours under the same temperature conditions as the decoke burn-out process.

In some embodiments, an anti-coking agent may also be included in the steam feed for the polish treatment or subsequent to the polish treatment. Many anti-coking agents are known to those skilled in the art. In some embodiments the anti-coking agent may be chosen from compounds of the formula RS_(n)R′ with n being the mean sulphur number ranging from 1 to 12 and R and R′ chosen from H and a linear or branched C1-C6 alkyl, cycloalkyl or aryl radicals. The anti-coking agent is added to the polish feed or a steam feed if the treatment is subsequent to the polish in an amount from 15 ppm to 2,500 ppm, for a period of time from 0.5 to 24, hours, preferably from about 1 to 6 hours at which time decreasing the dosing rate may begin.

Materials of the Furnace Tubes

The furnace tubes in hydrocarbon cracking furnaces are typically made of steel. The present disclosure is applicable to steels typically including at least 12 wt % Cr, preferably at least 16 wt % of Cr. The steel may be chosen from 304 stainless steel, 310 stainless steel, 315 stainless steel, 316 stainless steel, austenitic stainless steel and HP, HT, HU, HK, HW and HX stainless steel. In one embodiment the stainless steel, preferably heat resistant stainless steel typically includes from 13 to 50, preferably 20 to 50, most preferably from 20 to 38 wt % of Cr. The stainless steel may further include from 20 to 50, preferably from 25 to 50 most preferably from 25 to 48, desirably from about 30 to 45 wt % of Ni. The balance of the stainless steel is substantially iron.

The present disclosure may also be used with nickel and/or cobalt based extreme austenitic high temperature alloys (HTAs). Typically, the HTAs include a major amount of nickel or cobalt. Typically, the high temperature nickel-based alloys include from about 50 to 70, preferably from about 55 to 65 wt % of Ni; from about 20 to 10 wt % of Cr; from about 20 to 10 wt % of Co; and from about 5 to 9 wt % of Fe and the balance one or more of the trace elements noted below to bring the composition up to 100 wt %. Typically, the high temperature cobalt based alloys include from 40 to 65 wt % of Co; from 15 to 20 wt % of Cr; from 20 to 13 wt % of Ni; less than 4 wt % of Fe and the balance one or more of the trace elements noted below to bring the composition up to 100 wt %.

In some embodiments of the disclosure the substrate may further include at least 0.2 wt %, up to 3 wt % typically 1.0 wt %, up to 2.5 wt % preferably not more than 2 wt % of manganese from 0.3 to 2, preferably 0.8 to 1.6 typically less than 1.9 wt % of Si; less than 3, typically less than 2 wt % of titanium, niobium (typically less than 2.0, preferably less than 1.5 wt % of niobium) and all other trace metals; and carbon in an amount of less than 2.0 wt %.

The present disclosure may also be used with 35 wt % nickel and 45 wt % chromium based alloys with an amount of aluminum of up to 4% with a propensity to form an aluminum oxide layer or an alumina layer on the inner surface of a reactor or pass.

Furnace Tube Boundary Layer

The process of the present disclosure uses the practice of managing initial coke deposition in a cracking furnace's radiant coil internal surface or tube wall inner surface by maximizing the boundary layer turbulence of the furnace tube with the characteristics of the fluid being cracked, i.e. hydrocarbon plus diluent.

As the gas mix enters the furnace inlet tube at a high pressure, and as it increases in temperature along the length of the furnace tubes, the volumetric change in the gas results in a velocity increase of the gas as it travels to the outlet end of the furnace. The greater the velocity at the point of incipient cracking, the greater the surface turbulence minimizing the amount coke laydown onto the internal surface of a furnace tube of a newly built, rebuilt or de-coked hydrocarbon cracking furnace. The high gas velocity minimizes the thickness of the laminar flow of the boundary at the furnace tube wall providing a scouring effect upon the newly formed and deposited coke.

Using the Ideal Gas Law, desired gas velocity is determined by including the mass flow rate, molecular weight of the fluid, temperature of the fluid, the pressure, and area of the tube, as calculated thusly:

$\upsilon = {\frac{V}{A} = \frac{\frac{\frac{{\overset{.}{m}}_{1}}{{MW}_{1}}z_{1}{RT}}{3600P_{abs}} + \frac{\frac{{\overset{.}{m}}_{2}}{{MW}_{2}}z_{2}{RT}}{3600P_{abs}} + \ldots}{\pi r^{2}}}$

wherein v is gas velocity (m/s), V is gas volume (m³/s), A is pipe area (m²), m is mass flow rate of a gas (g/hr), MW is a molecular weight (g/mol) of a gas, z is the compressibility factor of a gas, R is the ideal gas constant (8.314 J·mol⁻¹·K⁻¹), T is the temperature (K), P_(abs) is the absolute pressure (Pa), and r is the pipe radius (m), and the subscripts 1 and 2 refer to gas 1 and gas 2, respectively. The ellipsis indicates there could be more than two components in the gas.

A feed gas chromatograph (GC) can provide the number average molecular weight of the hydrocarbon portion at the inlet of the furnace tube. For example, a hydrocarbon cracking furnace might have a front-end “sweetening” system, wherein about 65% of the total hydrocarbon feed to the hydrocarbon cracking furnace has gone through an amine contactor and has become saturated with water. This hydrocarbon can be blended with dry, recycled hydrocarbon so the water content's influence on the molecular weight of the hydrocarbon is not considered. The molecular weight of the dilution steam can be estimated as 18.015. The number average molecular weight of the fluid entering the furnace can then calculated thusly:

${MW}_{enter} = \frac{{\overset{.}{m}}_{hc} + {\overset{.}{m}}_{steam}}{{{\overset{.}{m}}_{hc}/\left( {MW}_{{hc}{from}{GC}} \right)} + {{\overset{.}{m}}_{steam}/(18.015)}}$

where {dot over (m)}_(hc) is the total mass flow of hydrocarbon, and {dot over (m)}_(steam) is the total mass flow of the dilution stream. An analogous method can be used to calculate the number average molecular weight of the gas at the outlet of the furnace tube.

The number average molecular weights can be varied by changing the composition of the fluid, such as by varying the steam to hydrocarbon ratio, or changing the types of hydrocarbons in the feed.

Start-Up Procedure

The start-up procedure after commissioning a furnace tube, or after a decoking has taken place, or for any other reason that the hydrocarbon cracking furnace is starting up can include a number of steps that are well known in the art.

The fluid velocity at the point of incipient cracking is not typically measured or controlled. If not measured, the fluid velocity at the point of incipient cracking can be estimated by measuring or calculating the number average molecular weight of the fluid at the inlet of the furnace tube, measuring or calculating the number average molecular weight of the fluid at the outlet of the furnace tube, calculating an average of the number average molecular weights of the fluids at the inlet and the outlet to estimate the number average molecular weight at the point of incipient cracking, measuring or calculating the fluid velocities and the fluid pressures at the inlet and outlet and a pressure drop of the fluid from the inlet to the outlet. Using the Ideal Gas Law as above, the fluid velocity at the point of incipient cracking can be calculated.

The fluid velocity at the point of incipient cracking is surprisingly a key variable in the operation of an industrial stream hydrocarbon cracking furnace, including the run length of the furnace before it needs to be shut down to perform a decoke. A hydrocarbon cracking furnace start-up in which the fluid velocity at the point of incipient cracking is maintained at a sufficient velocity from the start-up shall reduce the coke buildup on the inner walls of the furnace tube. The fluid velocity at the point of incipient cracking should be at least about 80 m/s to about 120 m/s, preferably at least about 85 m/s to about 115 m/s, preferably at least about 90 m/s to about 115 m/s, preferably at least about 90 m/s to about 110 m/s, preferably at least about 85 m/s to about 105 m/s, preferably at least about 90 m/s to about 105 m/s, preferably at least about 95 m/s to about 105 m/s. The fluid velocity at the point of incipient cracking, chosen to be 525° C., in this disclosure, should be maintained for at least five (5) 24-hour days, preferably 10 days, preferably 20 days.

The fluid velocity entering the radiant section can be greater than 295 ft/sec or 90 m/s, preferably greater than 311 ft/sec or 95 m/s, preferably greater than 340 ft/sec or 103.6 m/s. This velocity can be attained within five hours of introducing feed into the furnace tube, preferably with three hours, most preferably within less than 60 minutes.

EXAMPLES

The following example is presented for the purpose of illustrating selected embodiments of this disclosure; it being understood that the example presented does not limit the claims.

FIG. 5 shows the change in gas velocity (fluid velocity) over a 500 hundred day period, during which three consecutive runs of an industrial steam hydrocarbon cracking furnace were conducted. The three bars on the figure are plotted versus the x-axis showing 82 days (run 1), 39 days (run 2), and 343 days (run 3) for each of the three runs. The days indicates the number of cracking days, which is the number of days online, from hydrocarbon feed-in to hydrocarbon feed-out. The furnace was decoked after each run.

The left y-axis on FIG. 5 indicates the calculated value of the fluid velocity at the point of incipient cracking, the values calculated using the procedure described above.

An example calculation of the gas velocity at the 525° C. point of the coil for FIG. 5 , run 1, is shown as follows:

-   -   Hydrocarbon feed composition: >98 mol % ethane     -   Hydrocarbon feed MW: 29.96 g/mol     -   z for ethane=0.9915     -   Mass of hydrocarbon feed: 5,000,000 g/hr     -   Diluent composition: >99 mol % gaseous water     -   Diluent MW: 18.015 g/mol     -   z for diluent: 0.9613     -   Mass of diluent: 1,500,000 g/hr     -   Incipient cracking temperature: 525° C.+273.15=698.15 K     -   Pressure at incipient cracking: 300,000 Pa+92,000 Pa (ambient         pressure at 900 m above sea level)     -   R=8.31441 J·mol−1·K−1     -   Time t=3600 s/hr     -   Pi=3.141593

Therefore velocity=91.14 m/s.

For run 1, the gas velocity at the point of incipient cracking was only allowed to exceed 90 m/s for three days, resulting in a run length of 82 days before the hydrocarbon cracking furnace required decoking. For run 2, the gas velocity at the point of incipient cracking was only allowed to exceed 90 m/s for less than 1.5 days, resulting in a run length of 39 days before the hydrocarbon cracking furnace required decoking. Finally, for run 3, the gas velocity at the point of incipient cracking was allowed to exceed 90 m/s for greater than 20 days, resulting a run length of 343 days before the hydrocarbon cracking furnace required decoking.

These results demonstrate that run length of a hydrocarbon cracking furnace can be improved by controlling the fluid velocity at the point of incipient cracking, including maintaining a fluid velocity at the point of incipient cracking of at least 90 m/s, for at least the first 5 days after start-up.

INDUSTRIAL APPLICABILITY

The disclosure is related to operation of hydrocarbon cracking furnaces. Specifically, a start-up procedure is disclosed which allows longer run times before decoking is required. 

1. A procedure for starting up a hydrocarbon cracking furnace, wherein the hydrocarbon cracking furnace comprises a furnace tube, and the furnace tube comprises an inlet, an outlet, and a point of incipient cracking between the inlet and the outlet, wherein the procedure comprises: introducing a fluid comprising a hydrocarbon and steam to the inlet; maintaining a temperature of the fluid at the inlet at 25° C. to 225° C.; and controlling a fluid velocity at the point of incipient cracking to 90 m/s to 115 m/s for at least 24 hours by varying a flow rate of the fluid at the inlet, by varying a fluid property of the fluid at the inlet, or both.
 2. The procedure of claim 1, wherein the fluid velocity is controlled to 90 m/s to 110 m/s.
 3. (canceled)
 4. The procedure of claim 1, wherein the fluid velocity is controlled for at least 5 days.
 5. The procedure according to claim 1, wherein the fluid velocity is controlled for at least 10 days.
 6. (canceled)
 7. (canceled)
 8. The procedure of claim 1, wherein the hydrocarbon comprises one or more C1-C4 alkanes.
 9. The procedure of claim 1, wherein the fluid velocity is calculated using the Ideal Gas Law, based on an average of a number-average molecular weight of the fluid proximate the inlet and a number-average molecular weight of the fluid proximate the outlet.
 10. The procedure of claim 1, wherein the fluid velocity is controlled within 3 hours of the introduction of the fluid.
 11. (canceled)
 12. (canceled)
 13. The procedure of claim 1, wherein the temperature of the fluid at the inlet is maintained at 90° C. to 160° C.
 14. The procedure of claim 1, wherein a number average molecular weight of the fluid at the inlet is 16 g/mol to 60 g/mol.
 15. (canceled)
 16. The procedure of claim 1, wherein a pressure drop of the fluid from the inlet to the outlet is 275 kPa to 400 kPa.
 17. The procedure of claim 1, further comprising controlling a fluid velocity at the outlet to 190 m/s to 250 m/s.
 18. (canceled)
 19. The procedure of claim 1, further comprising controlling a fluid velocity at the outlet to 180 m/s to 260 m/s for at least 48 hours.
 20. (canceled)
 21. The procedure of claim 1, hydrocarbon comprises a mixture of a fresh hydrocarbon and a recycled hydrocarbon.
 22. The procedure of claim 8, wherein the C1-C4 alkane comprises ethane.
 23. The procedure of claim 1, wherein the hydrocarbon comprises naphtha, heavy aromatic distillate, aromatic hydrocarbons, or any mixture thereof.
 24. The procedure of claim 1, wherein the fluid velocity is controlled for an entire duration of a run length of the hydrocarbon cracking furnace.
 25. (canceled)
 26. (canceled)
 27. (canceled)
 28. (canceled)
 29. The procedure of claim 4, wherein a run length of operating the hydrocarbon cracking furnace after the starting-up procedure is extended by at least 15% as compared to the same procedure that does not include controlling the fluid velocity at the point of incipient cracking.
 30. The procedure of claim 29, wherein the run length is extended by at least 30%.
 31. The procedure of claim 1, wherein a run length of operating the hydrocarbon cracking furnace after the starting-up procedure is extended by at least 8 days as compared to the same procedure that does not include controlling the fluid velocity at the point of incipient cracking.
 32. The procedure of claim 31, wherein the run length is extended by at least 16 days. 